Pin feed devices have been utilized in many applications for positively and accurately advancing webs of indeterminate length. One conventional pin feed assembly is disclosed in detail in U.S. Pat. No. 3,930,601 issued Jan. 6, 1976 and assigned to the assignee of the present invention. The aforesaid patent discloses a pin feed tractor assembly for moving a web having uniformly spaced perforations along its opposite longitudinal sides. Pins movable by a pin feed belt enter into the perforations to provide positive drive for the web. The inner surface of the belt is provided with a plurality of teeth which cooperate to form spaced notches which are drivingly engaged by the teeth of a sprocket wheel.
The pins are arranged on a pin frame clamped to the belt by a fastening strap. A pair of such pin feed assemblies are arranged along opposite longitudinal edges of the web. The pair of pin feed assemblies differ from one another wherein the left-hand pin feed assembly may not be substituted to serve as a right-hand pin feed assembly. The assemblies are mounted upon a first guide shaft and a second drive shaft for holding the assemblies at the appropriate angular orientation. Means are provided for locking each pin feed assembly to the guide rod at any axial position therealong so as to maintain the pin feed assemblies in properly spaced fashion. Each assembly is provided with a sprocket having a rectangular shaped opening for receiving the drive shaft which preferably has a conforming rectangular shaped cross-section for positively driving both pin feed assemblies.
As the pins mounted upon the belt begin to move about a curve formed by the sprocket, those points along the outside of the curve move along a path of greater circumferential length than the points radially inward therefrom. This results in the pins moving further away from one another as the belt moves about the sprocket resulting in unnecessary stretching and even tearing of the web. As a further example, with tractor assemblies employed to move a laminated web assembly comprised of a multiplicity of plys, it is clear that the surface of the outermost ply moves along a path which is greater in length than the surface of the innermost ply causing undue stretching. In tractor assemblies in which four, five or six plys are being advanced, the outermost ply has been found to tear and hence conventional tractor assemblies of the type described hereinabove are useless under such conditions.
In addition to the above, it should be noted that the tractor assembly of the aforementioned U.S. Pat. No. 3,930,601 contains an extremely large number of components thus greatly complicating its assembly, disassembly and maintenance. Also the pin frames and cooperating belt provide a bulky and somewhat clumsy arrangement. The pins being offset from the belt have a tendency to undergo twisting thereby affecting the uniform feeding of the web.